In optical telecommunication systems, one of the many difficulties encountered is the chromatic dispersion of light signals propagating over long distances in optical fibers. The chromatic dispersion in non-dispersion-shifted optical fiber is nominally 17 ps/nm/km in the 1550 nm telecommunication window, but this value changes as a function of the wavelength: its value changes by about 2 ps/nm/km between 1530 nm and 1565 nm. Several single-channel dispersion compensators based on Fiber Bragg gratings (FBGs) have been proposed, and although this solution was demonstrated to be an appropriate solution for compensating the chromatic dispersion in a single WDM channel, for multi-channel systems, the spectral variation of the chromatic dispersion must be taken into account, especially for data transmission systems operating at high rates such as 10 and 40 Gbit/s. There is therefore a need for a broadband dispersion compensator that compensates for the chromatic dispersion but also for its spectral variation. This feature is often referred to as the slope compensation.
Fiber Bragg gratings are a well established technology for the fabrication of components for optical telecommunications, especially for WDM. Basically, a Bragg grating allows light propagating into an optical fiber to be reflected back when its wavelength corresponds to the grating's Bragg wavelength, related to its period. A chirped Fiber Bragg Grating, in which the grating period varies as a function of the position along the fiber, represents a well known solution for compensating the chromatic dispersion of an optical fiber link (F. Ouellette, “Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides,” Opt. Lett., 12, pp.847-849, 1987; R. Kashyap, “Fiber Bragg gratings,” Academic Press, 458p., 1999). Such a grating compensates for the accumulated dispersion since the group delay varies as a function of the wavelength. An appropriate grating can be fabricated such that the wavelength dependence of its group delay is just the opposite of that of the fiber link. Different solutions based on FBGs have been proposed for broadband dispersion compensation but most of them do not include the slope compensation.
Referring to M Durkin et al. “1 m long continuously written fibre Bragg grating for combined second- and third-order dispersion compensation”, Electron. Lett. 33, pp 1891-1893 (1997) and J. F. Brennan et al. in BGPP 1999, pp.35-37, ultra-long FBGs, up to 10 m, have been demonstrated for dispersion compensation over a large bandwidth. However, such devices suffer from high group delay ripples. The group delay of a compensator based on ultra-long FBGs is schematically illustrated in FIG. 1 (prior art). The chromatic dispersion the device compensates for is given by the slope of the group delay. The example shown in FIG. 1 has a dispersion of −1250 ps/nm and thus compensates for the chromatic dispersion accumulated over a 73 km long fiber link.
Sampled FBGs and Moiré FBGs have also been proposed in U.S. Pat. No. 5,384,884 (KASHYAP et al.) noteworthy for multi-channel dispersion compensation (see for example A. E. Willner, et al., “Tunable compensation of channel degrading effects using nonlinearly chirped passive fiber Bragg gratings,” IEEE J. of Selected Topics in Quantum Electron., 5, pp.1298-1311 (1999), U.S. Pat. No. 5,982,963 (FENG et al.); A. V. Buryak et al., “Novel multi-channel grating designs”, Proceedings of BGPP 2001; and M. Ibsen et al., “Chirped moiré fiber gratings operating on two-wavelength channels for use as dual-channel dispersion compensators,” IEEE Photon. Technol. Lett., 10, pp.84-86, (1998)) in which the sampling function replicates a given dispersion function (M. Ibsen et al, “Sinc-sampled fiber Bragg gratings for identical multiple wavelength operation,” IEEE Photon. Technol. Lett., 10, pp.842-844, 1998). As a result, all the channels are identical and the resulting device cannot compensate for the dispersion slope. The group delay of such a compensator based on a sampled FBG is schematically illustrated in FIG. 2 (prior art). An approach for multi-channel slope compensation has been proposed based on interleaved sampled Bragg gratings in W H Loh et al. “Sampled fiber grating based dispersion slope compensator”, >>Photonics Technol. Lett. 11, no 10, pp 1280-1282 (1999). The theoretical approach is expected to suffer from significant practical difficulties associated with the control of the many micro-grating structures.
Single-channel non-linearly chirped FBGs have been proposed for narrowband dispersion slope compensation (J. A. R. Williams et al., “Fiber Bragg grating fabrication for dispersion slope compensation,” IEEE Photon. Technol. Lett., 8, pp.1187-1189, 1996). In order to achieve operation over a broader range, multi-channel non-linearly chirped FBGs were proposed (Y. Xie et al., “Tunable compensation of the dispersion slope mismatch in dispersion-managed systems using a sampled nonlinearly chirped FBG,” IEEE Photon. Technol. Lett., 12, pp.1417-1419, 2000). This last approach allows a tuning of the dispersion but the spectral duty factor is limited to about 25%.